Alpha-substituted polymers such as methacrylates, methacrylonitriles, and .alpha.-methylstyrene are thermally unstable and are known to depolymerize to their corresponding monomers at temperatures greater than 230.degree. C. Above 300.degree. C., poly(methyl methacrylate) (PMMA) depolymerizes rapidly at high conversions (&gt;95%). Typical extruding and molding temperatures for such polymers are 200.degree.-250.degree. C. Significant depolarization to the respective monomers would occur in this temperature range, affecting the safety of the operation as well as the properties of the product. Polymethacrylates, and in particular PMMA, are the alpha-substituted polymers most widely used in commercial applications. In order to broaden the range of applications for these polymers it is critical to improve their thermal stability.
In the manufacture of graft copolymers comprising a backbone of a propylene polymer material, to which is grafted poly(methyl methacrylate), small amounts of nonmethacrylate monomers such as methyl acrylate, butyl acrylate and styrene are typically copolymerized with the methyl methacrylate to improve thermal stability, since polymers of these monomers are much more stable to heat and undergo degradation at relatively higher temperatures. However, addition of these monomers affects the mechanical properties of the graft copolymers as well as the molecular weight and grafting efficiency.
As reported by Nishimoto et al., Polymer, 32, 1275 (1991), methacrylate monomers have been used with methyl methacrylate to improve compatibility with other functional polymers such as polycarbonates, but the authors concluded that they did not improve thermal stability.
Thus there is a need for method to increase the thermal stability of graft copolymers that include polymerized alpha-substituted acrylates compared with that achievable with the nonmethacrylate comonomers currently used for this purpose.